Luminescent glasses that are transmissive to light from the near ultraviolet (200-300 nm) to the near infrared (3000 nm) and that can be formed into clear glass plates and fibers (See R. F. Reade, U.S. Pat. No. 3,654,172), lenses (See T. Takahashi, et al, U.S. Pat. No. 4,259,587), or ground glass are used to detect various forms of electromagnetic, electron beam, particle beam or nuclear radiation by luminescing under the proper excitation conditions. The threshold for luminescence in glass as in other inorganic luminescent materials is typically above an excitation energy of approximately 2.5 eV (See for example Blasse, G. and Bril, A. "Characteristic Luminescence," Philips Technical Review, Vol. 31, No. 10, 304, 1970).
A method for using luminescent glass is to view the emission in real-time employing a video camera. Important characteristics of the glass in this mode include high absorption of the excitation radiation, high luminescence responsivity, prompt emission, low afterglow, and a linear response over a wide range of excitation intensity. The luminescent glass shall also emit light within the wavelength sensitivity range of the recording camera and shall maintain the spatial resolution of the primary image. Particular applications for the glass in this mode include detecting x-rays, or gamma rays for the purpose of diagnostic medical or industrial imaging (See for example Bossi, R. H., Oien, C. T., "Real-Time Radiography" RPT #DE82-005603, UCRL-53091, LLNL, CA) or for tracking high energy particles (See for example, Ruchti, R. et al, "Scintillating Glass, Fiber Optic Plate Imaging System for Tracking Applications in High Energy Physics Experiments", J. Luminescence Vol. 31-32, Part 1 and 2, Dec. 1984, Proceedings of the 1984 International Conference on Luminescence, Madison, WI, USA, Aug. 13-17, 1984 p. 721-723).
Luminescent glasses are of particular interest in real-time x-ray radiography. Traditionally, in real-time x-ray radiography, a visible image is generated by polycrystalline x-ray-to-light conversion screen that is monitored by a low light level TV camera. A similar screen can also be employed in an x-ray image amplifier tube where the visible image generated by the phosphor is further electro-optically intensified prior to collection by a TV camera. This screen is typically composed of polycrystalline x-ray phosphor particles embedded in a binder, and both are supported on a thin mylar sheet as taught by Buchanan, R. A. et al. U.S. Pat. No. 3,725,704. Although many commercial polycrystalline phosphor screens have a strong luminescene signal under x-rays (as illustrated by Buchanan, R. A., et al in U.S. Pat. No. 4,297,584), the image resolution is limited by the light scatter among individual phosphor particles in the screen and the image contrast can be limited by their relatively low x-ray absorption efficiencies. Furthermore, light scatter in thicker, more absorbing screens will trap the light embedded in the sublayers of the screen to result in a reduced light signal and therefore a reduced contrast perceptibility. The binder in the screen, which can be present in as much as 50 percent by weight has the added disadvantages of being inert and provides no means for transferring energy absorbed within to luminescent centers in the crystalline phosphor particles. These same problems exist when these phosphor screens are used as film intensifier screens in radiographic film cassettes.
Luminescent glass plates and fiber optic scintillating plates offer solutions to the problems imposed by polycrystalline phosphor screens. Such glass plates do not degrade resolution by transverse light scattering within the glass. Secondly, increased x-ray absorption and improved noise statistics can be realized by the use of thicker (1/4") glass plates where light from the entire cross section of the plate can reach the recording medium without being trapped. Thirdly, no binder is required, the theoretically all the x-ray energy absorbed can be transferred to the luminescent centers in the glass. Finally, luminescent glass plates are more resistant to environmental attack, for example, scratches, chemical reaction, and abrasion.
The advantage of using thick glass plates is particularly important for industrial radiography where high energy x-rays (100 kev-15 mev) are used and where the thick glass plates will absorb and convert a higher portion of the photon flux than the more x-ray transparent thin polycrystalline phosphor screens. Illustrative uses for high energy luminescent glasses include a stand alone luminescent glass plate viewed by a low level TV camera forming a real time radiographic system, a fiber-optic scintillating plate incorporated in an x-ray image amplifying fluorescopy tube, and as a fiber optic scintillating plate for intensification of film in a radiographic film cassette.
Although luminescent glass detection screens have shown promise for x-ray radiographic applications (U.S. Pat. No. 3,654,172), they haven't been widely used because of phosphorescence problems including signal instability (i.e., background buildup) and low-luminescence responsivity. Those that have been used are activated with terbium oxide, but suffer from these drawbacks.
The effects of phosphorescence and background buildup are believed to be caused by radiation produced free electrons and holes that are trapped in defect centers in the glass structure. Such defect centers can consist of vacancies, interstitial atoms or other types of charge imbalance. Over time, these trapped carriers are released by thermal and electromagnetic (i.e., x-rays) stimulation, and radiatively recombine at luminescence centers to result in phosphorescence. In real-time systems, the buildup of the background is a result of the gradual filling of traps. With increasing illumination, intensity or duration, deeper and deeper traps become filled, and eventually emptied and hence the long decay component is increased and the background level increases. If the background levels are significant, this could lead to increased noise and image lag. The problem becomes apparent in making observations of very weak signals following very strong signals. This problem is not limited to luminescent glasses but has also arisen with some polycrystalline phosphor materials in real time imaging systems (See for example Torr, M. R. "Persistence of Phosphor Glow in Microchannel Plate Image Intensifiers," Applied Optics, Vol. 24, No. 6, 793, 1985).
Commercially available terbium activated silicate luminescent glasses, including those developed under U.S. Pat. No. 3,654,172, have a strong phosphorescence after only moderate exposures to x-rays. Applicants have found that this phosphorescence is manifested as a signal increase of as much as 130% after 3000 R and 180% after 25,000 R of its initial value under continuous illumination at 250 kVp.
Other commercially available terbium activated silicate luminescent glasses demonstrate similar, if not more pronounced buildup behavior under similar x-ray conditions. The applicants are unaware of any commercial terbium activated x-ray luminescent silicate glasses that do not have this problem. The problem is common to any prior silicon oxide glass that contains terbium activation. For example, terbium activated fused silicon oxide glass demonstrates a severe phosphorescence and signal buildup under moderate x-ray exposures.
The prior art does not give any guidance for reducing phosphorescence in these materials while retaining or improving luminescence responsivity under x-rays.
Another aspect limiting the use of existing luminescent glasses for the application of x-ray imaging is their low luminescence efficiency (gram for gram) compared to polycrystalline phosphor materials. The known prior art host materials allow only a low level of terbium activation before the onset of concentration quenching (where increased levels of activator do not increase light output and can reduce responsivity by increasing the probability of trapping in the glass). There exists a need for new host materials permitting increased levels of terbium oxide before the onset of concentration quenching with a resulting improvement in light output.